Throughout this Specification, the phrase "ring laser" will denote a rod laser having a ring cavity configuration, the phrase "YAG rod laser" will denote any solid state laser rod (e.g., a Nd:YAG laser rod), and the phrase "YAG ring laser" will denote any ring laser resonator including a solid state laser rod (e.g., a ring laser resonator including a Nd:YAG rod). One convenient parameter that will be used in the Specification for describing the quality of a laser output beam is: M.sup. =(.pi./4L) (2B) (2W), where L is the laser wavelength, 2B is the full angle far-field beam divergence, and 2W is the laser beam focal spot diameter.
Rod lasers have been employed in a variety of commercial applications. YAG ring lasers (of the type including a Nd:YAG rod) have been proposed, for example in A. R. Clobes, et al., "Single-frequency Travelingwave Nd:YAG Laser," Applied Physics Letters, Vol. 21, pp. 265-267, (1972).
For many applications, such as drilling and cutting, it would be desirable to produce a high power, high quality output beam from a laser. Such an output beam would preferably have not only high power but also low far-field beam divergence and low laser beam focal spot diameter.
However, for a number of reasons it is difficult to produce such a high power, high quality output beam. An important reason why it is difficult to produce a high power, high quality output beam is that the quality of a laser output beam is dependent upon the power dissipated in the laser rod. The power dependent phenomena of thermal focal lensing, rod birefringence, and thermally dependent rod end curvature (resulting in spherical and non-spherical aberration) all contribute to the power dependence of beam quality by affecting the laser rod focal length. Several techniques have been proposed for compensating for the effect of individual ones of these phenomena on the output beam quality of linear rod lasers, in contrast with ring lasers.
For example, to compensate for rod birefringence in a YAG rod laser having linear cavity configuration, it has been proposed that a 90.degree. quartz rotator be inserted between two collinear YAG rods in the linear cavity. See W. C. Scott, "Birefringence Compensation and TEM.sub.00 Mode Enhancement in a Nd:YAG Laser," Applied Physics Letters, Vol. 18, No. 1, pp. 3-4 (1971). This technique relies on the fact that the effect of birefringence induced by one rod will be cancelled by the other rod. However, the technique has the disadvantage that it requires use of two appropriately aligned rods in a linear resonator.
In order to compensate for thermal focal lensing, it has been proposed that an intra-cavity telescope, consisting of a pair of lenses with variable inter-lens spacing, be included in the beam path of a rod laser. As the output, beam power (and hence the power dissipated in the rod) varies, the spacing between the lenses of the telescope is varied to compensate for thermally induced variation in the rod focal length. It is conventional to employ a servo positioning mechanism to vary the telescope lens spacing. Measured signals indicative of output beam power have been used as feedback signals for such servo positioning mechanisms.
In order to compensate for thermally dependent rod focal power, inclusion of a positive spherical lens in a linear rod laser beam path (to partially compensate for spherical aberration) has been proposed. Similarly, inclusion of a tilted thin planoconvex lens in a linear rod laser beam path to compensate for astigmatism has been proposed. See W. Koechner, Solid-State Laser Engineering. Springer-Verlag New York Inc, 1976, pp. 364-365.
The present invention is a ring laser capable of producing a high quality output beam over a wide and varying output power range, without the need for more than one laser rod, and with enhanced laser system component lifetime.